Reconfigurable intelligent surfaces (RISs) have been proposed as a promising technology for future wireless communication systems and extensively investigated in recent years [1]}, [2]}. Specifically, an RIS is an array composed of massive passive elements. It can construct favorable wireless propagation environment between transmitters and receivers by adaptively adjusting the phase-shift of the reflected electromagnetic (EM) waves which impinge on the surface. By deploying RIS in wireless systems, the channel power gain can be effectively improved and the communication QoS can be enhanced without significant additional power consumptions.
